Fish make a natural group of animals, because they have similar methods of reproduction and breathing, and structures such as gills, fins and scales.
Like the reptiles and the amphibians, the fish are poikilothermic or cold-blooded, and this can bring special problems for fish in polar waters.
Fish are found in both fresh and salt waters all over the world, though some fish can survive out of water for a short while, enough to move from pool to pool.
Some of the early fish developed their fins into something not unlike legs, and moved onto the shore, giving rise to the earliest four-legged amphibians.
Bony fish show a wide variety of adaptations in their structures and behaviour. Some fish also show a limited ability to learn from their experiences.
Bony fish are generally able to adjust their buoyancy, in much the same way that submarines do by adding or removing gases from a swim bladder.
Sharks and rays have cartilage for their skeletons and no bone, and they have a number of other common characteristics, like the lack of a swim bladder.
Sharks have heterocercal tails which produce lift as they swim. This, with the angling of their pectoral fins, keeps the negative buoyancy sharks from sinking.
Some sharks reproduce in unexpected ways, some of them nurturing eggs within their body and actually producing live young, while others lay eggs.
Fish rely on a variety of senses, including sound (vibration detection), smell (taste), sight, and electric senses which are effective in muddy water.